When fabricating thin film transistors (TFT) on a glass substrate, the TFTs at different positions generally have non-uniformity in electrical parameters such as threshold voltage, mobility, and the like, due to non-uniformity in a fabrication process or parameter drifts resulted from being at high temperature and high pressure for a long time. This non-uniformity may be translated into current differences and brightness differences in a display device, which will be perceived by human eyes. Therefore, it is necessary to solve this non-uniform phenomenon by designing various compensation techniques. The compensation methods are generally divided into internal compensation methods and external compensation methods.